The present invention generally relates to automatic frequency control methods and circuits. More particularly, the present invention relates to an automatic frequency control method for correcting an error between a received carrier frequency and a local frequency in a demodulator which uses the 2.sup.n -phase phase shift keying (PSK) modulation system, and to an automatic frequency control circuit which employs such a method.
Recently, communication is frequently made via a satellite, and the value of satellite communications has increased due to the emergence of very small aperture terminals (VSATs). When communicating by satellite, frequency deviation becomes a problem. The frequency shift of the local oscillation frequency of a receiver due to a temperature change, the Doppler effect and the like are the causes of such frequency deviation. Frequency deviation similarly occurs in mobile communications.
Accordingly, the frequency (reference carrier frequency) of a local oscillator of a demodulator must follow the frequency deviation. An automatic frequency control (AFC) loop is provided for this purpose.
FIG. 1 generally shows an example of a conventional AFC circuit of a demodulator which uses the general 2.sup.n -phase PSK modulation system, where n is an integer greater than or equal to two. An input signal is passed through a mixer 11 and a bandpass filter 12 to convert the input signal into an intermediate frequency (IF) signal. This IF signal is separated into digital I-axis and Q-axis signals by a quadrature wave detector 13. The frequencies of the I-axis and Q-axis signals are multiplied by 2.sup.n by a 2.sup.n -frequency multiplier 14 so as to eliminate unwanted modulation data. Furthermore, a frequency discriminator 20 discriminates the frequency deviation, and a digital loop filter (lowpass filter) 21 limits the frequency band (noise elimination). Thereafter, a digital-to-analog (D/A) converter 22 converts an output of the digital loop filter 21 into an analog signal, to generate a local frequency control voltage for a local oscillator which is provided with respect to the mixer 11 or the quadrature wave detector 13. FIG. 1 shows a case where a local oscillator 25 is provided with respect to the quadrature wave detector 13, but as an alternative, it is possible to provide the local oscillator 25 with respect to the mixer 11. In the latter case, no oscillator is provided with respect to the quadrature wave detector 13.
The frequency discriminator 20 includes delay circuits 15 and 16 for respectively delaying the I-axis and Q-axis signals by one symbol .tau. (clock), a multiplier 17 for multiplying the I-axis signal and the delayed I-axis signal, a multiplier 18 for multiplying the Q-axis signal and the delayed Q-axis signal, and a subtractor 19 for calculating an output difference between the multipliers 17 and 18.
The delay operations of the delay circuits 15 and 16 of the frequency discriminator 20 are carried out in response to a clock which is received from a symbol timing reproducing circuit (not shown) at a timing when an eye pattern of the received signal is most open.
The output amplitude of the frequency discriminator 20 changes depending on the frequency deviation, and a frequency pull-in is carried out by the AFC operation.
However, according to the conventional AFC circuit, the frequency is multiplied by 2.sup.n in the frequency multiplier by subjecting the amplitude information to an operation, and in addition, the error information of the frequency is extracted using the multipliers. For this reason, there is a problem in that the circuit scale of the AFC circuit inevitable becomes large.